I am using OpenCart 3.0.2.0, and I would like to add my private key to be able to use the SSL, the problem is that OpenCart accepts only the 1024 bit key and my website is using 2048 bit key and i cannot generate a 1024 bit key

The Encryption Key field in the OpenCart settings is just for use in OpenCart's built in encryption functions. It has nothing to do with SSL. To get SSL working you need to install the certificate on your hosting. Or are you trying to achieve something else?

PHP compared to SSL may only support a solid 256 bits out of 2048 when using algorithm other than SSL which SSL do support a higher tolerance for the solidness of the resourceful algorithm. However, you'd need to contact your host in order to install such level of SSL certificate that can only be added to your domain by a relative SSL certificate provider compared to the self-signed SSL certificates.

For instance, one of the reasons why the certificate X.509 V3 was also abolished this year is because SSL V3 provides better protection.

The most generated errors being found on Opencart forum originates from contributed programming. The increased post counters are caused by redundancies of the same solutions that were already provided prior.

The Encryption Key field is generated randomly on install, there should be no need to change it. If you do want change it, because it doesn't look random enough or you've use it elsewhere, etc. Make sure it's long and as unique and random as possible.